Field of the Invention
This invention relates to methods and apparatus for mounting an article on an adherent site on a substrate and, more particularly, to adhering a plurality of very small articles to the sites with a precise orientation and location.
2. Description of the Prior Art
Beam-lead semiconductor devices are usually bonded individually to their respective thin-film circuits on a substrate. In most cases, this individual bonding will suffice. However, in some cases it is necessary to bond several hundred like devices to as many sites on a substrate. It is desirable to bond the semiconductor devices en masse for economic reasons in such cases.
Mass bonding of the devices to thin-film circuit sites on a substrate requires orienting and locating the devices in an exact array, i.e., positioning them precisely, to match the sites on the substrate. Further, it requires aligning the substrate with the precisely positioned array of devices and mounting the devices on their sites on the substrate in condition for bonding.
Mass bonding of crossovers is disclosed in the prior art by J. A. Burns, "Bonded Crossovers for Thin Film Circuits," Proceedings 1971 21st Electronics Components Conference, IEEE, N.Y. In this method the crossovers are manufactured in an array on a polymide film in the same position they will have in the thin-film circuit. The crossovers are manufactured on the film in the location and orientation needed by the circuit so that no individual positioning of the crossovers is required. After bonding, the polymide film is easily removed by dissolving its adhesive.
This technique cannot be used for beam-lead semiconductor devices because they cannot be made in place on a polymide film. It is necessary, therefore, to manufacture the beam-lead devices separately and subsequently place them in the precise position on the substrate for bonding.
The devices are square and approximately one-sixteenth of an inch from tip to tip of the leads which project cantilever fashion from each side of square semiconductor bodies. The leads are approximately 0.7 mil thick, 4 mils wide and 8 mils long. The devices are, therefore, minute, and fragile and weigh very little. This makes them very difficult to position, especially without damaging them. For example, a method of centering a semiconductor slice by lowering it into a shallow dish or saucer on a cushion of air is not suitable for the tiny devices because they would blow away.
This has been overcome by lowering the devices into a cavity and introducing a fluid to "float" the devices to a central position by virtue of the surface tension and meniscus of the fluid.
Also, prior art methods terminate the cavity in a pocket which is large enough to accommodate the largest part. Thus, the smallest device is not oriented or located as precisely as the largest one.
Once the devices are positioned in the cavities, the substrate to which the devices are to be bonded, must be aligned precisely with the cavities so that the devices may be mounted on, i.e., adhered to the sites on the substrate. The sites to which the devices are adhered are first coated with a hydrocarbon material such as eicosane. However, the substrate must be heated and then cooled rapidly, for economical reasons, to secure the devices. Once the devices are adhered to the substrate they may be compliant bonded by methods known in the art.